Levothyroxine is a thyroid hormone product, commercially available in 10 different dose strengths. However, only 5 dose strengths are permissible in the present formulary at NIH. Some clinicians feel that the availability of all dose formulations will allow for a safer, more efficacious titration of dose. This randomized control study will evaluate (over a 4 year period) thyroid function, prescribing patterns, patient compliance, frequency of thyroid function studies and clinic visits, medication accuracy, and inventory cost with and without formulary restrictions on levothyroxine dose strengths (5 vs. 10 dose strengths). The study population will consist of 75 NIH physicians from the NICHD and NIDDK clinics who prescribe levothyroxine. Preliminary two-year interim results suggest that patients receiving suppression therapy may benefit from the availability of the additional dosage formulations. Laboratory assessment of patients receiving replacement therapy did not differ significantly between restricted and non- restrictive levothyroxine formulary groups. The second two-year study phase, utilizing trough (blood sampling for thyroid function tests prior to patients receiving their daily levothyroxine dose) laboratory values will be completed by June of 1993. Results will provide quality assurance and therapeutic drug monitoring information, which can be used to fulfill requirements suggested by the Joint Commission on Accreditation of Hospitals (JCAH).